The importance of the steroid hormone microenvironment within cells is now recognised in studies on endocrine-related neoplasms such as breast cancer. This focuses attention on enzymes which control the intracellular levels of estradiol-17 beta (E2). One such enzyme, estrogen sulfotransferase, which converts E2 to inactive E2-3 sulfate, has now been shown to be regulated by estrogen in MCF-7 human mammary cancer cells. Hydroxysteroid sulfotransferase, which sulfurylates the adrenal-derived estrogen 5-androstene-3 beta,17 beta-diol, is also under estrogen control. Evidence is provided which shows that one function of these enzymes may involve elimination of estrogen from the cell following processing of the ligand-charged estrogen receptor (ER).

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